Acoustic resonator structure comprising a bridge

ABSTRACT

An acoustic resonator comprises a first electrode a second electrode and a piezoelectric layer disposed between the first and second electrodes. The acoustic resonator further comprises a reflective element disposed beneath the first electrode, the second electrode and the piezoelectric layer. An overlap of the reflective element, the first electrode, the second electrode and the piezoelectric layer comprises an active area of the acoustic resonator. The acoustic resonator also comprises a bridge adjacent to a termination of the active area of the acoustic resonator.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application under 37 C.F.R.§1.53(b) of U.S. patent application Ser. No. 12/490,525 filed on Jun.25, 2009, naming John Choy, et al. as inventors. Priority under 35U.S.C. §120 is claimed from U.S. patent application Ser. No. 12/490,525,and the entire disclosure of U.S. patent application Ser. No. 12/490,525is specifically incorporated herein by reference.

BACKGROUND

In many electronic applications, electrical resonators are used. Forexample, in many wireless communications devices, radio frequency (rf)and microwave frequency resonators are used as filters to improvereception and transmission of signals. Filters typically includeinductors and capacitors, and more recently, resonators.

As will be appreciated, it is desirable to reduce the size of componentsof electronic devices. Many known filter technologies present a barrierto overall system miniaturization. With the need to reduce componentsize, a class of resonators based on the piezoelectric effect hasemerged. In piezoelectric-based resonators, acoustic resonant modes aregenerated in the piezoelectric material. These acoustic waves areconverted into electrical waves for use in electrical applications.

One type of piezoelectric resonator is a Film Bulk Acoustic Resonator(FBAR). The FBAR has the advantage of small size and lends itself toIntegrated Circuit (IC) manufacturing tools and techniques. The FBARincludes an acoustic stack comprising, inter alia, a layer ofpiezoelectric material disposed between two electrodes. Acoustic wavesachieve resonance across the acoustic stack, with the resonant frequencyof the waves being determined by the materials in the acoustic stack.

FBARs are similar in principle to bulk acoustic resonators such asquartz, but are scaled down to resonate at GHz frequencies. Because theFBARs have thicknesses on the order of microns and length and widthdimensions of hundreds of microns, FBARs beneficially provide acomparatively compact alternative to known resonators.

Desirably, the bulk acoustic resonator excites onlythickness-extensional (TE) modes, which are longitudinal mechanicalwaves having propagation (k) vectors in the direction of propagation.The TE modes desirably travel in the direction of the thickness e.g.,z-direction) of the piezoelectric layer.

Unfortunately, besides the desired TE modes there are lateral modes,known as Rayleigh-Lamb modes, generated in the acoustic stack as well.The Rayleigh-Lamb modes are mechanical waves having k-vectors that areperpendicular to the direction of TE modes, the desired modes ofoperation. These lateral modes travel in the areal dimensions (x, ydirections of the present example) of the piezoelectric material. Amongother adverse effects, lateral modes deleteriously impact the quality(Q) factor of an FBAR device. In particular, the energy of Rayleigh-Lambmodes is lost at the interfaces of the FBAR device. As will beappreciated, this loss of energy to spurious modes is a loss in energyof desired longitudinal modes, and ultimately a degradation of theQ-factor.

FBARs comprise an active area, and connections to and from the activearea can increase losses, and thereby degrade the Q factor. For example,in transition regions between the active area and the connections,defects may form in the piezoelectric layer during fabrication. Thesedefects can result in acoustic loss, and as a result, a reduction in theQ factor.

What is needed, therefore, are an acoustic resonator structure andelectrical filter that overcomes at least the known shortcomingsdescribed above.

SUMMARY

In accordance with a representative embodiment, an acoustic resonatorcomprises: a first electrode; a second electrode; a piezoelectric layerdisposed between the first and second electrodes; and a reflectiveelement disposed beneath the first electrode, the second electrode andthe piezoelectric layer. An overlap of the reflective element, the firstelectrode, the second electrode and the piezoelectric layer defines anactive area of the acoustic resonator. The first electrode substantiallycovers the reflective element, and the piezoelectric layer extends overan edge of the first electrode. The acoustic resonator also comprises abridge adjacent to a termination of the active area of the acousticresonator, and the bridge overlaps a portion of the first electrode.

In accordance with another representative embodiment, a film bulkacoustic resonator (FBAR) comprises: a first electrode; a secondelectrode; a piezoelectric layer disposed between the first and secondelectrodes; and a cavity disposed beneath the first electrode, thesecond electrode and the piezoelectric layer. An overlap of the cavity,the first electrode, the second electrode and the piezoelectric layerdefines an active area of the acoustic resonator. The first electrodesubstantially covers the cavity, and the piezoelectric layer extendsover an edge of the first electrode. The FBAR also comprises a bridgeadjacent to a termination of the active area of the acoustic resonator.The bridge overlaps a portion of the first electrode.

In accordance with yet another representative embodiment, a filterelement comprises an acoustic resonator. The acoustic resonatorcomprises: a first electrode; a second electrode; a piezoelectric layerdisposed between the first and second electrodes; and a reflectiveelement disposed beneath the first electrode, the second electrode andthe piezoelectric layer. An overlap of the reflective element, the firstelectrode, the second electrode and the piezoelectric layer defines anactive area of the acoustic resonator. The first electrode substantiallycovers the reflective element, and the piezoelectric layer extends overan edge of the first electrode. The acoustic resonator also comprises abridge adjacent to a termination of the active area of the acousticresonator, and the bridge overlaps a portion of the first electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The illustrative embodiments are best understood from the followingdetailed description when read with the accompanying drawing figures. Itis emphasized that the various features are not necessarily drawn toscale. In fact, the dimensions may be arbitrarily increased or decreasedfor clarity of discussion. Wherever applicable and practical, likereference numerals refer to like elements.

FIG. 1 shows a cross-sectional view of an acoustic resonator inaccordance with a representative embodiment.

FIG. 2 shows a top view of an acoustic resonator in accordance with arepresentative embodiment.

FIG. 3 shows a graph of the Q-factor versus a spacing between theair-bridge and the lower electrode of an FBAR in accordance with arepresentative embodiment.

FIG. 4 shows a graph of the effective coupling coefficient (kt²) versusa spacing between the air-bridge and the lower electrode of an acousticresonator in accordance with a representative embodiment.

FIG. 5 shows a cross-sectional view of an FBAR in accordance with arepresentative embodiment.

FIG. 6 shows a graph of the parallel impedance (Rp) for FBARs, includingcertain FBARs of representative embodiments.

FIG. 7 shows a graph of the effective coupling coefficient (kt²) forFBARs, including certain FBARs of representative embodiments.

DEFINED TERMINOLOGY

It is to be understood that the terminology used herein is for purposesof describing particular embodiments only, and is not intended to belimiting. The defined terms are in addition to the technical andscientific meanings of the defined terms as commonly understood andaccepted in the technical field of the present teachings.

As used in the specification and appended claims, the terms ‘a’, ‘an’and ‘the’ include both singular and plural referents, unless the contextclearly dictates otherwise. Thus, for example, ‘a device’ includes onedevice and plural devices.

As used in the specification and appended claims, and in addition totheir ordinary meanings, the terms ‘substantial’ or ‘substantially’ meanto within acceptable limits or degree. For example, ‘substantiallycancelled’ means that one skilled in the art would consider thecancellation to be acceptable.

As used in the specification and the appended claims and in addition toits ordinary meaning, the term ‘approximately’ means to within anacceptable limit or amount to one having ordinary skill in the art. Forexample, ‘approximately the same’ means that one of ordinary skill inthe art would consider the items being compared to be the same.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, specific details are set forth in order to provide athorough understanding of illustrative embodiments according to thepresent teachings. However, it will be apparent to one having ordinaryskill in the art having had the benefit of the present disclosure thatother embodiments according to the present teachings that depart fromthe specific details disclosed herein remain within the scope of theappended claims. Moreover, descriptions of well-known apparati andmethods may be omitted so as to not obscure the description of theillustrative embodiments. Such methods and apparati are clearly withinthe scope of the present teachings.

FIG. 1 is a cross-sectional view of an acoustic resonator 100 inaccordance with an illustrative embodiment. Illustratively, the acousticresonator 100 is an FBAR structure. The acoustic resonator 100 comprisesa substrate 101. A first electrode 102 is disposed over the substrate101. A piezoelectric layer 103 is disposed over the first electrode 102.A second electrode 104 is disposed over the first electrode 102. Thepiezoelectric layer 103 has a first surface in contact with the firstelectrode 102 and a second surface in contact with the second electrode104. The first and second electrodes 102, 104 include an electricallyconductive material and provide an oscillating electric field in they-direction, which is the direction of the thickness of thepiezoelectric layer 103. In the present illustrative embodiment, they-axis is the axis for the TE (longitudinal) mode(s) for the resonator.

The piezoelectric layer 103 and first and second electrodes 102, 104 aresuspended over a cavity 105 formed by selective etching of the substrate101. The cavity 105 may be formed by a number of known methods, forexample as described in commonly assigned U.S. Pat. No. 6,384,697 toRuby, et al. The region of overlap of the first and second electrodes102, 104, the piezoelectric layer 103 and the cavity 105 is referred toas the active area of the acoustic resonator 100. Accordingly, theacoustic resonator 100 is a mechanical resonator, which can beelectrically coupled via the piezoelectric layer 103. Other suspensionschemes that foster mechanical resonance by FBARs are contemplated. Forexample, the acoustic resonator 100 can be located over a mismatchedacoustic Bragg reflector (not shown) formed in or on the substrate 101.This type of FBAR is sometimes referred to as a solid mount resonator(SMR) and, for example, may be as described in U.S. Pat. No. 6,107,721to Lakin, the disclosure of which is specifically incorporated into thisdisclosure by reference in its entirety.

By contrast, an inactive area of the acoustic resonator 100 comprises aregion of overlap between first electrode 102, or second electrode 104,or both, and the piezoelectric layer 103 not disposed over the cavity105 or other suspension structure. As described more fully below, it isbeneficial to the performance of the resonator to reduce the area of theinactive region of the acoustic resonator 100 to the extent practical.

When connected in a selected topology, a plurality of acousticresonators 100 can act as an electrical filter. For example, theacoustic resonators 100 may be arranged in a ladder-filter arrangement,such as described in U.S. Pat. No. 5,910,756 to Ella, and U.S. Pat. No.6,262,637 to Bradley, et al. The electrical filters may be used in anumber of applications, such as in duplexers.

The acoustic resonator 100 may be fabricated according to knownsemiconductor processing methods and using known materials.Illustratively, the acoustic resonator 100 may be fabricated accordingto the teachings of U.S. Pat. Nos. 5,587,620; 5,873,153; 6,384,697;6,507,983; and 7,275,292 to Ruby, et al.; and 6,828,713 to Bradley, et.al. The disclosures of these patents are specifically incorporatedherein by reference. It is emphasized that the methods and materialsdescribed in these patents are representative and other methods offabrication and materials within the purview of one of ordinary skill inthe art are contemplated.

The acoustic resonator 100 also comprises a bridge 106 provided on aside of the acoustic resonator 100 that connects to a contact 107. Thecontact 107 is connected to a signal line (not shown) and electroniccomponents (not shown) are selected for the particular application ofthe acoustic resonator 100. This portion of the acoustic resonator 100is often referred to as the interconnection side of the acousticresonator 100. By contrast, the second electrode 104 terminates at aposition 108 over the cavity 105 in order to minimize the inactive areaof the acoustic resonator 100 as described below. The position 108opposes the interconnection side of the acoustic resonator 100.

The bridge 106 comprises a gap 109 formed beneath a portion of thesecond electrode 104. Illustratively, and as described below, afterremoval of a sacrificial layer (not shown) provided in the formation ofthe gap 109, the gap 109 comprises air. However, the gap 109 maycomprise other materials including low acoustic impedance materials,such as carbon (C) doped SiO₂, which is also referred as Black-diamond;or dielectric resin commercially known as SiLK; or benzocyclobutene(BCB). Such low acoustic impedance materials may be provided in the gap109 by known methods. The low acoustic impedance material may beprovided after removal of sacrificial material used to form the gap 109(as described below), or may be used instead of the sacrificial materialin the gap 109, and not removed.

In a representative embodiment, the bridge 106 is formed by providing asacrificial layer (not shown) over the first electrode 102 and a portionof the piezoelectric layer 103 on the interconnection side and formingthe second electrode 104 over the sacrificial layer. Illustratively, thesacrificial material comprises phosphosilicate glass (PSG), whichillustratively comprises 8% phosphorous and 92% silicon dioxide.Subsequent layers such as the piezoelectric layer 103 and the secondelectrode 104 are deposited, sequentially, upon the PSG until the finalstructure is developed. Notably a seed layer (not shown) may be providedover the first electrode 102 before depositing the piezoelectric layer103, and a passivation layer (not shown) may be deposited over thesecond electrode 104. After the formation of the structure comprisingthe bridge 106, the PSG sacrificial layer is etched away illustrativelywith hydrofluoric acid leaving the free-standing bridge 106. In arepresentative embodiment, the sacrificial layer disposed in the cavity105 and the sacrificial layer beneath the bridge 106 are removed in thesame process step, with the latter leaving the gap 109 comprising air.

The piezoelectric layer 103 comprises a transition 110 formed during theformation of the piezoelectric layer 103 over the first electrode 102and the substrate 101. The piezoelectric layer 103 at the transition 110often comprises material defects and voids, particularly structuraldefects such as lattice defects and voids. These defects and voids canresult in losses of acoustic energy of the mechanical waves propagatingin the piezoelectric material. As should be appreciated, acoustic energyloss results in a reduction in the Q-factor of the acoustic resonator100. However, and as described below, by separating the second electrode104 from the piezoelectric layer 103 in a region 111 of the gap 109where the transition 110 occurs, the portion of the active region of theacoustic resonator 100 necessarily does not include the transition 110of the piezoelectric layer 103 that includes the defects and voidstherein. As a result, acoustic losses due to the defects and voids inthe piezoelectric layer 103 at the transition 110 are reduced and theQ-factor is improved compared to known resonators, such as known FBARs.

Additionally, and beneficially, the bridge 106 provides an acousticimpedance mismatch at the boundary of the active region on theinterconnection side of the acoustic resonator 100. This acousticimpedance mismatch results in the reflection of acoustic waves at theboundary that may otherwise propagate out of the active region and belost, resulting in energy loss. By preventing such losses, the bridge106 results in an increased Q-factor in the acoustic resonator 100.Moreover, the termination of the second electrode 104 at position 108terminates the active region of the acoustic resonator 100 and reduceslosses by creating an acoustic impedance mismatch. This also provides animprovement in the Q-factor.

In addition to terminating the active region of the acoustic resonator100 before the transition 110, the bridge 106 also reduces the area ofan inactive region of the acoustic resonator 100. The inactive region ofthe FBAR 100 creates a parasitic capacitance, which in an equivalentcircuit is electrically in parallel with the intrinsic capacitance ofthe active region of the FBAR. This parasitic capacitance degrades theeffective coupling coefficient (kt²), and therefore it is beneficial toreduce the parasitic capacitance. Reducing the area of the inactiveregion reduces the parasitic capacitance, and thereby improves theeffective coupling coefficient (kt²).

FIG. 1 also shows a first line 112 and a second line 113 separated by adistance ‘d.’ The distance ‘d’ represents the distance between the edgeof the first electrode 102 at the first line 112 (arbitrarily selectedas x=0 for this discussion) to the beginning point of the bridge 106 atthe second line 113. As the position of the second line 113 becomeslarger (more negative), the Q-factor increases. The effective couplingcoefficient (kt²) also increases with separation of first and secondlines 112, 113 to an extent. Thus, the selection of a particulardistance ‘d’ results in an improvement in Q due to reduced acousticlosses due to the reduction in the inactive FBAR area and theimprovement in kt² from the reduction of the parasitic capacitance.

FIG. 2 shows a top view of the acoustic resonator 100 of FIG. 1.Notably, the cross-sectional view of the acoustic resonator 100 shown inFIG. 1 is taken along the line 1-1. The second electrode 104 of thepresent embodiment is apodized to reduce acoustic losses. Furtherdetails of the use of apodization in acoustic resonators may be found incommonly owned U.S. Pat. No. 6,215,375 to Larson III, et al; or incommonly owned U.S. Pat. No. 7,629,865 entitled “Piezoelectric ResonatorStructures and Electrical Filters” filed May 31, 2006, to Richard C.Ruby. The disclosures of U.S. Pat. Nos. 6,215,375 and 7,629,865 arespecifically incorporated herein by reference in their entirety.

The fundamental mode of the acoustic resonator 100 is the longitudinalextension mode or “piston” mode. This mode is excited by the applicationof a time-varying voltage to the two electrodes at the resonantfrequency of the acoustic resonator 100. The piezoelectric materialconverts energy in the form of electrical energy into mechanical energy.In an ideal FBAR having infinitesimally thin electrodes, resonanceoccurs when the applied frequency is equal to the velocity of sound ofthe piezoelectric medium divided by twice the thickness of thepiezoelectric medium: f=v_(ac)/(2*T), where T is the thickness of thepiezoelectric medium and v_(ac) is the acoustic phase velocity. Forresonators with finite thickness electrodes, this equation is modifiedby the weighted acoustic velocities and thicknesses of the electrodes.

A quantitative and qualitative understanding of the Q of a resonator maybe obtained by plotting on a Smith Chart the ratio of the reflectedenergy to applied energy as the frequency is varied for the case inwhich one electrode is connected to ground and another to signal, for anFBAR resonator with an impedance equal to the system impedance at theresonant frequency. As the frequency of the applied energy is increased,the magnitude/phase of the FBAR resonator sweeps out a circle on theSmith Chart. This is referred to as the Q-circle. Where the Q-circlefirst crosses the real axes (horizontal axes), this corresponds to theseries resonance frequency f_(s). The real impedance (as measured inOhms) is R_(s). As the Q-circle continues around the perimeter of theSmith chart, it again crosses the real axes. The second point at whichthe Q circle crosses the real axis is labeled f_(p), the parallel oranti-resonant frequency of the FBAR. The real impedance at f_(p) isR_(p).

Often it is desirable to minimize R_(s) while maximizing R_(p).Qualitatively, the closer the Q-circle “hugs” the outer rim of the Smithchart, the higher the Q-factor of the device. The Q-circle of an ideallossless resonator would have a radius of one and would be at the edgeof the Smith chart. However, as noted above, there are energy lossesthat impact the Q of the device. For instance, and in addition to thesources of acoustic losses mentioned above, Rayleigh-Lamb (lateral orspurious) modes are in the x,y dimensions of the piezoelectric layer103. These lateral modes are due to interfacial mode conversion of thelongitudinal mode traveling in the z-direction; and due to the creationof non-zero propagation vectors, k_(x) and k_(y) for both the TE modeand the various lateral modes (e.g., the S0 mode and the zeroth andfirst flexure modes, A0 and A1), which are due to the difference ineffective velocities between the regions where electrodes are disposedand the surrounding regions of the resonator where there are noelectrodes.

Regardless of their source, the lateral modes are parasitic in manyresonator applications. For example, the parasitic lateral modes coupleat the interfaces of the resonator and remove energy available for thelongitudinal modes and thereby reduce the Q-factor of the resonatordevice. Notably, as a result of parasitic lateral modes and otheracoustic losses sharp reductions in Q can be observed on a Q-circle ofthe Smith Chart of the S₁₁ parameter. These sharp reductions in Q-factorare known as “rattles” or “loop-de-loops,” which are shown and describedin commonly owned U.S. Pat. No. 7,280,007, referenced below.

As described more fully in U.S. Pat. Nos. 6,215,375 and 7,629,865, theapodized first and second electrodes 102, 104 cause reflections of thelateral modes at the interfaces of the resonator to interferenon-constructively, and therefore reduce the magnitude of lateral modeswhich otherwise result in more viscous energy dissipation. Beneficially,because these lateral modes are not coupled out of the resonator anddeveloped to higher magnitude, energy loss can be mitigated with atleast a portion of the reflected lateral modes being converted tolongitudinal modes through mode conversion. Ultimately, this results inan overall improvement in the Q-factor.

FIG. 3 shows a graph of the Q-factor versus a spacing (‘d’ in FIG. 1)between the bridge 106 and the first electrode 102 of acoustic resonator100 in accordance with a representative embodiment. When the spacing ‘d’is selected to be zero, second line 113 is aligned with first line 112,and the region 111 is eliminated. When the spacing ‘d’ is selected to bepositive, the second line 113 now has a positive x-coordinate (i.e.,second line 113 is located to the right of first line 112 in FIG. 1). Ineither case, the acoustic losses due to defects in the transition 110and a comparative increase in the area of the inactive region of theacoustic resonator 100 combine to result in a Q-factor that iscomparatively low. By contrast, when the spacing d is selected to be‘negative’ the inactive area is decreased (i.e., second line 113 islocated to the left of first line 112 in FIG. 1), with the bridge 106and region 111 comprising a comparatively increased dimension. As can beseen in region 302 of FIG. 3, the Q-factor increases to a maximum valueat 303. As should be appreciated, the reduction in the inactive area ofthe acoustic resonator 100 on the interconnection side of the acousticresonator 100, and the forming of the region 111 results in a decreasein losses due to defects and an acoustic impedance mismatch at theboundary of the active region of the acoustic resonator 100 at theinterconnection side of the acoustic resonator 100.

FIG. 4 shows a graph of the effective coupling coefficient (kt²) versusa spacing (‘d’ in FIG. 1) between the air-bridge (e.g., 106) and thelower electrode (e.g., 102) of an acoustic resonator 100 in accordancewith a representative embodiment. When the spacing is selected to bezero, second line 113 is aligned with first line 112 and the region 111is eliminated. When the spacing ‘d’ is selected to be positive, thesecond line 113 now has a positive x-coordinate (i.e., second line 113is to the right of first line 112 in FIG. 1). In either case, the areaof the inactive region results in an increase in the parasiticcapacitance. This increased parasitic capacitance provides acomparatively reduced effective coupling coefficient (kt²) as shown inregion 401 of FIG. 4. By contrast, when the spacing d is selected to be‘negative’ (i.e., second line 113 is to the left of first line 112 inFIG. 1) the inactive area is decreased, with the bridge 106 and region111 increasing in dimension. This results in a reduction in theparasitic capacitance, and a comparative increase in the effectivecoupling coefficient (kt²) as shown in region 402 of FIG. 4. A maximumvalue of the effective coupling coefficient (kt²) is reached at 403 inFIG. 4. In region 404 of FIG. 4, the effective coupling coefficient(kt²) begins to drop. Likely, this is due to the fact that region 111further increases over cavity 105, resulting in an increase in the areaof the parasitic capacitor formed by the bridge 106 and the gap 109,thus increasing the parasitic capacitance in parallel with the intrinsiccapacitance of the acoustic resonator 100. Moreover, as region 111increases within cavity 105, the active resonator area also decreases,resulting in a decrease in kt². Accordingly, in region 404, theeffective coupling coefficient (kt²) decreases as region 111 increases.As should be appreciated from the discussion of FIGS. 3 and 4, theselection of the distance ‘d’ in FIG. 1 allows for the selection of anacceptable increase in both the Q-factor and the effective couplingcoefficient (kt²).

FIG. 5 is a cross-sectional view of an acoustic resonator 500 inaccordance with an illustrative embodiment. The acoustic resonator 500,which is illustratively an FBAR, shares many common features with theacoustic resonator 100 described previously. Many of these commondetails are not repeated in order to avoid obscuring the presentlydescribed embodiments.

The acoustic resonator 500 comprises a selective recess 501 (oftenreferred to as an ‘innie’) and a frame element 502 (also referred to asan ‘outie’). The recess 501 and frame element 502 provide an acousticmismatch at the perimeter of the second electrode 104, improve signalreflections and reduce acoustic losses. Ultimately, reduced lossestranslate into an improved Q-factor of the device. While the recess 501and the frame element 502 are shown on the second electrode 104, thesefeatures may instead be provided on the first electrode 102, orselectively on both the first and second electrodes 102, 104. Furtherdetails of the use, formation and benefits of the recess 501 and theframe element 502 are found for example, in commonly owned U.S. Pat. No.7,280,007 entitled “Thin Film Bulk Acoustic Resonator with a Mass LoadedPerimeter” to Feng, et al.; and commonly owned U.S. Patent ApplicationPublication 20070205850 entitled “Piezoelectric Resonator Structure andElectronic Filters having Frame Elements” to Jamneala, et al. Thedisclosures of this patent and patent application publication arespecifically incorporated herein by reference.

FIG. 6 shows a graph of the impedance Rp at parallel resonance forFBARs, including certain acoustic resonators of representativeembodiments. Notably, Rp of a known resonator is shown at 601; Rp of anacoustic resonator comprising the bridge 106 (e.g., acoustic resonator100 of FIG. 1) is shown at 602; Rp of an acoustic resonator comprisingrecesses and frame elements (with no bridge) is shown at 603; and Rp ofan acoustic resonator comprising both the bridge 106, and the recess 501and the frame element 502 (e.g., acoustic resonator 500 of FIG. 5) isshown at 604.

The known resonator typically has Rp (shown at 601) of approximately2000 ohm. The addition of a recess and a frame element in a known FBARincreases Rp by approximately 1 kΩ as shown at 602. Similarly, theaddition of the bridge 106 but not the recess 501 and or the frameelement 502 increases Rp by approximately 1 kΩ as shown at 603. However,when adding combined features of the bridge 106 and the recess 501 andframe element 502, the overall parallel resonance Rp improves by nearly3 kΩ (over that of the known resonator) as shown at 604. Accordingly,the bridge 106, and the recess 501 and frame element 502 provide asynergistic increase in the parallel resonance Rp, as is evident by acomparison of 601 and 604 in FIG. 6. It is beneficial to havecomparatively high Rp in FBARs to provide comparatively low insertionloss and comparatively ‘fast’ roll off filters comprising such FBARs.

As is known, although the use of recesses such as recess 501 results ina comparatively small increase in the effective coupling coefficient(kt²), there can be a degradation in the effective coupling coefficient(kt²) as a result of the frame elements and recesses. In someapplications, it may be useful to mitigate this degradation, even thoughthe improvement in the Q-factor may not be as great. For instance, it isknown that the bandwidth of an FBAR filter is related to kt². As such, adegradation of kt² can reduce the bandwidth of the FBAR filter. Certainrepresentative embodiments described presently provide somewhat of atrade-off of an acceptable Q-factor and an acceptable degradation ofkt².

FIG. 7 shows a graph of the effective coupling coefficient (kt²) forFBARs, including certain acoustic resonators of representativeembodiments. The effective coupling coefficient (kt²) of a knownresonator is shown at 701; the effective coupling coefficient (kt²) ofan acoustic resonator comprising bridge 106 (e.g., acoustic resonator100 of FIG. 1) is shown at 702; the effective coupling coefficient (kt²)a known acoustic resonator comprising a recess and a frame element (butwith no bridge) is shown at 703; and the effective coupling coefficient(kt²) of an acoustic resonator comprising both a bridge 106, and therecess 501 and the frame element 502 (e.g., acoustic resonator 500 ofFIG. 5) is shown at 704.

The effective coupling coefficient (kt²) of the known resonator isapproximately 5.3 as shown at 701. The addition of the bridge 106improves the effective coupling coefficient (kt²) to 5.4 as shown at702. However, adding recesses and frame elements and no airbridge willresult in an effective coupling coefficient (kt²) of approximately 5.15as shown at 703. Finally, incorporating the bridge 106, the recess 501and the frame element 502 result in an effective coupling coefficient(kt²) (shown at 704) that is substantially the same as the known FBAR.Thus, the positive impact on the effective coupling coefficient (kt²)from the bridge 106 must be contrasted with the negative impact ofrecesses and frame elements on the effective coupling coefficient (kt²).

In accordance with illustrative embodiments, acoustic resonators forvarious applications such as in electrical filters are described havinga bridge. One of ordinary skill in the art appreciates that manyvariations that are in accordance with the present teachings arepossible and remain within the scope of the appended claims. These andother variations would become clear to one of ordinary skill in the artafter inspection of the specification, drawings and claims herein. Theinvention therefore is not to be restricted except within the spirit andscope of the appended claims.

1. An acoustic resonator, comprising: a first electrode; a secondelectrode; a piezoelectric layer disposed between the first and secondelectrodes; a reflective element disposed beneath the first electrode,the second electrode and the piezoelectric layer, an overlap of thereflective element, the first electrode, the second electrode and thepiezoelectric layer defining an active area of the acoustic resonator,wherein the first electrode substantially covers the reflective element,and the piezoelectric layer extends over an edge of the first electrode;and a bridge adjacent to a termination of the active area of theacoustic resonator, wherein the bridge overlaps a portion of the firstelectrode.
 2. An acoustic resonator as claimed in claim 1, furthercomprising an electrical connection to one of a plurality of sides ofthe second electrode, wherein the bridge is disposed between theconnection and the one side of the second electrode.
 3. An acousticresonator as claimed in claim 1, wherein adjacent to termination of theactive area of the acoustic resonator, the piezoelectric layer comprisesa transition comprising defects.
 4. An acoustic resonator as claimed inclaim 3, wherein the second electrode does not contact the transition.5. An acoustic resonator as claimed in claim 1, wherein the bridgecomprises a gap.
 6. An acoustic resonator as claimed in claim 5, whereinthe gap comprises a region between the second electrode and thepiezoelectric layer.
 7. An acoustic resonator as claimed in claim 6,wherein adjacent to termination of the active area of the acousticresonator, the piezoelectric layer comprises a transition comprisingdefects, and the transition is disposed beneath the region of the gap.8. An acoustic resonator as claimed in claim 1, wherein the secondelectrode comprises an upper surface with a side and a recess isdisposed along the side.
 9. An acoustic resonator as claimed in claim 1,wherein the second electrode comprises an upper surface with a side, anda frame element is disposed along the side.
 10. A film bulk acousticresonator (FBAR), comprising: a first electrode; a second electrode; apiezoelectric layer disposed between the first and second electrodes; acavity disposed beneath the first electrode, the second electrode andthe piezoelectric layer, an overlap of the cavity, the first electrode,the second electrode and the piezoelectric layer defining an active areaof the acoustic resonator, wherein the first electrode substantiallycovers the cavity, and the piezoelectric layer extends over an edge ofthe first electrode; and a bridge adjacent to a termination of theactive area of the acoustic resonator, wherein the bridge overlaps aportion of the first electrode.
 11. An FBAR as claimed in claim 10,further comprising an electrical connection to one of a plurality ofsides of the second electrode, wherein the bridge is disposed betweenthe connection and the one side of the second electrode.
 12. An FBAR asclaimed in claim 10, wherein adjacent to termination of the active areaof the acoustic resonator, the piezoelectric layer comprises atransition comprising defects.
 13. An FBAR as claimed in claim 12,wherein the second electrode does not contact the transition.
 14. AnFBAR as claimed in claim 10, wherein the bridge comprises a gap.
 15. AnFBAR as claimed in claim 14, wherein the gap comprises a region betweenthe second electrode and the piezoelectric layer.
 16. An FBAR as claimedin claim 15, wherein adjacent to termination of the active area of theacoustic resonator, the piezoelectric layer comprises a transitioncomprising defects, and the transition is disposed beneath the region ofthe gap.
 17. An FBAR as claimed in claim 10, wherein the secondelectrode comprises an upper surface with a side and a recess isdisposed along the side.
 18. An FBAR as claimed in claim 10, wherein thesecond electrode comprises an upper surface with a side, and a frameelement is disposed along the side.
 19. An FBAR as claimed in claim 20,further comprising a low acoustic impedance material beneath the bridge.20. A filter element comprising an acoustic resonator, the acousticresonator comprising: a first electrode; a second electrode; apiezoelectric layer disposed between the first and second electrodes; areflective element disposed beneath the first electrode, the secondelectrode and the piezoelectric layer, an overlap of the reflectiveelement, the first electrode, the second electrode and the piezoelectriclayer defining an active area of the acoustic resonator, wherein thefirst electrode substantially covers the reflective element, and thepiezoelectric layer extends over an edge of the first electrode; and abridge adjacent to a termination of the active area of the acousticresonator, wherein the bridge overlaps a portion of the first electrode.21. An acoustic wave resonator, comprising: (a) a substrate; (b) anacoustic mirror formed in or on the substrate, having a first edge andan opposite, second edge; (c) a dielectric layer formed on the substratesuch that the dielectric layer is substantially in contact with thefirst and second edges of the acoustic mirror; (d) a first electrodeformed on the acoustic mirror, having a first end portion and anopposite, second end portion defining a body portion therebetween,wherein at least one of the first and second end portions is formedextending to the dielectric layer; (e) a piezoelectric layer formed onthe first electrode, having a body portion, a first end portion and asecond end portion oppositely extending from the body portion onto thedielectric layer; and (f) a second electrode formed on the piezoelectriclayer, having a first portion situated on the body portion of thepiezoelectric layer, and a second portion extending from the firstportion such that the junction of the first portion and the secondportion locates between the first and second edges of the acousticmirror and the second portion of the second electrode and the first endportion of the piezoelectric layer define an air gap therebetween. 22.The acoustic wave resonator of claim 21, wherein the air gap is filledwith a dielectric material.
 23. The acoustic wave resonator of claim 22,wherein the dielectric material comprises carbon (C) doped silicondioxide (SiO₂), crosslinked polyphenylene polymer (SiLK), orbenzocyclobutene (BCB).
 24. The acoustic wave resonator of claim 21,wherein the second portion of the second electrode comprises a convexbridge.
 25. The acoustic wave resonator of claim 21, wherein the firstand second end portions of first electrode are formed to have a verticalprofile.
 26. The acoustic wave resonator of claim 21, Wherein the firstend portion of the first electrode extends beyond the first edge of theacoustic mirror and is situated on the dielectric layer.
 27. Theacoustic wave resonator of claim 26, wherein the first edge of theacoustic mirror and the junction of the first portion and the secondportion of the second electrode define a first distance, d₁.
 28. Theacoustic wave resonator of claim 27, wherein the junction of the bodyportion and the first end portion of the first electrode and thejunction of the first portion and the second portion of the secondelectrode define a second distance, d₂.
 29. An acoustic wave resonator,comprising: (a) a substrate having a top surface; (b) an acoustic mirrorformed on the top surface of the substrate or in the substrate, having afirst edge and an opposite, second edge; (c) a first electrode formed onthe acoustic mirror, having a end portion; (d) a piezoelectric layerformed on the first electrode; and (e) a second electrode formed on thepiezoelectric layer, wherein at least one of the first electrode and thesecond electrode and the piezoelectric layer define an air gap in aregion that overlaps the end portion of the first electrode.
 30. Theacoustic wave resonator of claim 29, further comprising a dielectriclayer formed on the substrate such that the dielectric layer issubstantially in contact with the first and second edges of the acousticmirror.
 31. The acoustic wave resonator of claim 29, wherein the air gapis filled with a dielectric material.
 32. The acoustic wave resonator ofclaim 31, wherein the dielectric material comprises carbon (C) dopedsilicon dioxide (SiO₂), crosslinked polyphenylene polymer (SiLK), orbenzocyclobutene (BCB).